In case of quenching a tubular workpiece as a thin cylindrical tube, for instance the thickness of which is 3 millimeters, there is a problem that the workpiece is very likely to warp and the diameter thereof may be subjected to change. Methods by which a workpiece is quenched with some constraining force have been already known as means for preventing warping during quenching. However, because of being thin, there are such problems that a portion of the workpiece applied the constraining force becomes concave and that the workpiece is elongated in a longitudinal direction by the constraining force of a diametral direction.
There is a Japanese laid-open patent No. Sho 57-192221 as a prior art for quenching a thin tubular workpiece wherein the present invention relates. According to this method, a tubular workpiece is rotatably supported by both of the end faces, and is made expansible and contractible in the axial direction in heating and cooling processes. And then, groups of rotary disks which are brought into contact with the workpiece at intervals in the longitudinal direction are pressed to the workpiece from two orthogonal directions, thereby it is prevented from warping and at the same time from dimensional changes by controlling cooling rate. But in this method unless the pressure is sufficiently controlled, the diameter of the workpiece may be partially constricted, because the rotary disks are brought into contact with the workpiece at intervals in the longitudinal direction.
Furthermore, in Japanese laid-open patent No. Sho 54-67504, it is disclosed that a round bar is quenched while it is put among three rolls having a length identical thereto and it is rotated. However, in a case where this method is used for a thin tube, such a phenomenon that the tube diameter is reduced and the tube length is elongated may occur under usual pressure of rolls for preventing the tube from warping.
Still furthermore, as for a bar workpiece of different shape such as a rack is formed on its side, the above method cannot be adopted wherein the workpiece is rotated while quenching. In this case press quenching has been conventionally applied as a method for preventing warping during quenching. According to this method, all or part of a workpiece, which is heated to appointed hardening temperature, is pressed in dies having concave portions of appointed shape, and is placed in a cooling liquid tank. Recently, there are cases where a bar workpiece which a rack is formed therealong is made hollow and tubular for the purpose of making automobiles lighter. In these cases, there may be a possibility for such a workpiece being collapsed by compressive force in the above press quenching.
Formerly, the inventors of the present invention have improved the press quenching and have invented an apparatus wherein dies are brought into contact with each other when they are closed, therefore no pressure is exerted to the workpiece in the dies (International laid-open patent: 94/14985, EPC laid-open patent No. 0628641). In this apparatus, no pressure is exerted to a workpiece when the workpiece is still soft at high temperature just after the beginning of the quenching and warping is prevented by constraining force of dies when the workpiece is caused to warp due to cooling.
It is a theme of the present invention that no collapse nor change in diameter occur, wherein constrained quenching is carried out in order to prevent warping of the tubular workpiece. A method and an apparatus of this invention must be applicable not only to a tubular workpiece of cylindrical tube, but also that of a different shape which has a flat side extending therealong and so on. Also it is an object of the present invention to provide a method and an apparatus which are superior to the former invention in capacity to prevent warping during quenching.